Scoring is never a good thing. Depending on how bad you might be able to get away with using it.
If it was me and tight on cash i would at a minimum replace the piston and rings. I would also look at EBay and see what i could get a decent used usable cylinder for but that is a risk also.
In the end depending on the scoring your probably ok with a new piston and rings.

 

It's not so much a tight on cash thing as a "how much do you put into a 22 year old 4 wheeler that gets used twice a year" thing. I think I'll see if I can find a local shop that can give it a look over and if necessary bore it a bit farther and get me a bigger piston. I have bought used for the 2 little 50's after realizing the new aftermarket were junk (they literally weighed half of the original, they were too thin and I'm sure would not have lasted), but on a bigger one like this I'm worried about the condition of a used one. Now to find a fair machine shop in the area.

 

Got ya.
Only you know what you wanna do with it.
Im the type that depending on the rest of the machine was decent and usable i would dump some cash into it but i like the older stuff myself as long as parts are not an issue.
My current ride is a 2004 700 that sat for years with a bad clutch. Last licensed in 2008 and was pretty much gave to me as long as i bought the other one for 1K.

 

Quick update on this one in case anyone else runs into a similar problem (exhaust gets loud due to leaking out of the bottom of the cylinder because of sheared bolts). I had very minor scuffs on the cylinder (couldn't feel them, only could see them), they came out with a quick hone. I imagine they would have gotten bad quickly if I had not stopped riding right away. I replaced the piston as it had scuffs as well. New Piston, Rings, studs, gaskets, etc. Quad fired up on the first attempt and is running good. I haven't finished breaking it in so I can't say it's running as strong as it used to, but it seems to be fine.

So in summary, broken cylinder studs on one of these 400cc 2 strokes does not mean something catastrophic happened inside the engine. I'm assuming mine just broke from fatigue, possibly over tightened the last time it was rebuilt approx 20 years ago.

 

Good job on fixing it and thanks for reporting back!

 

Reopening this old problem. On my 2nd break in attempt (yes this has been years in the making), it over heated. I noticed an antifreeze smell then pretty quickly the radiator cap popped. I shut it off and towed it back to the house and noticed some antifreeze on the engine but could not tell where it’s coming from, hoping it just boiled out of the reservoir.
I wasn’t driving so slowly that It should have over heated from lack of air flow. I did not think to check the fan at the time but that’s on my list now, but I don’t think it’s going to be so simple. I read an old post about air in the system and need to get that out of there so the coolant can cycle through? Ideas? I’m really not willing to give up on this thing.